4.2 Microbiome of Ceratitis capitata
Despite a wide range of host plants and large distances among collection
sites, many microbial taxa were identified in both the native and
introduced ranges of the medfly. The microbiome composition was highly
similar across the biological replicates and the sampling sites, except
for the specimens collected in Brazil. The microbiome of all samples was
dominated by the phylum Proteobacteria followed by a few other
predominant phyla: Firmicutes, Bacteroidetes, and Actinobacteria, which
is consistent with the significant phyla previously observed in
association with tephritids (Deutscher, Chapman, Shuttleworth, Riegler,
& Reynolds, 2019; Morrow, Frommer, Royer, Shearman, & Riegler, 2015;
Nikolouli et al., 2020). It is important to note that in this study, we
used individual mature adults collected in wild conditions, while
previous studies were performed on pooled dissected guts of larvae (De
Cock et al., 2019; De Cock et al., 2020), adult guts (Nikolouli et al.,
2020) or adults collected from infected fruits that emerged in
laboratory conditions (Malacrino et al., 2018). Despite the technical
differences, these studies have defined Proteobacteria as the
predominant phylum, albeit at a higher abundance (≥ 80%) compared to
our study (60.4%). Besides this, the abundance of the phylum Firmicutes
in those studies was lower (≤ 10%) compared to our results (18.7%).
Differences observed in abundances for these phyla might be explained by
differences in the sample source and/or life stage of the medflies
analysed. At different taxonomic levels, the prevalence of an
unclassified Enterobacteriaceae genus (Gammaproteobacteria) was
expected because it is known to dominate the first set of maternally
inherited microbiota, which is vertically transmitted during oviposition
in tephritids (Aharon et al., 2013; Behar, Yuval, & Jurkevitch, 2005;
Deutscher et al., 2019).
The microbiome abundance and composition remained similar to the
native range across different host plants and some distant localities
despite the low number of ASVs detected in South Africa. Furthermore,
the microbiome structure in the introduced range presented similar
numbers of ASVs among Spain, Israel, Australia and Colombia, with
significant differences in their microbiome composition only found for
Spain and Israel. These results of the microbiome structure mirror our
population genetic analysis, where we found genetic similarities among
all the populations in the introduced range, except Brazil. Therefore,
we suggest that these microbial communities are stable regardless of the
distances, host plants and differences in environmental conditions, also
reflecting the interconnectivity between these localities. In this
context, similarities in microbiome composition were found between Spain
and Australia, which were also highly connected populations in our DAPC
(both populations appeared in the same genetic cluster) and population
structure analysis. On this basis, the connectivity of the medfly
populations might facilitate a bacterial exchange/transmission across
them, suggesting the attainment of an essential microbiome set over time
that successfully serves the medfly’s polyphagous nature (Gruber et al.,
2019). This could represent a solid contributing factor to the species’
invasive success by ensuring a universal way of feeding on various
plants in introduced ranges.
A particular case is the unique microbiome composition in the Brazilian
population. A recognised factor that influences microbiome composition
is feeding on different plant species (Malacrino et al., 2018). However,
in this study, Brazil and the native South Africa were collected from
the same host plant, Guava. Consequently, we suggest that the somewhat
isolated flies of Brazilian populations acquired new bacteria from the
new environment. Also, exclusively in Brazil, we found in high relative
abundance Acinetobacter (phylum Proteobacteria) that was
previously described in larvae of medfly (De Cock et al., 2019;
Malacrino et al., 2018). The genus Acinetobacter is associated
with plant defence suppression mechanisms in polyphagous insects, which
is known to help insects to detoxify phenolic glycosides in vitro(Mason, Couture, & Raffa, 2014), although some other bacteria, which
are difficult to isolate in culture, may contribute to the metabolism of
this metabolite.
Unclassified Burkholderiaceae in Brazil were previously described only
in medfly adults collected in Italy (Malacrino et al., 2018). The
presence of these bacteria has been associated with nitrogen fixation
(i.e. diazotroph microbes), which is an essential mechanism for the
fly’s nutrition, development, and reproduction (Behar et al., 2005;
Raza, Yao, Bai, Cai, & Zhang, 2020). Most insect microbiomes are
maintained by strict vertical transmission, however, noteworthy is the
case of the bean bug Riptortus pedestris (Heteroptera: Alydidae),
where it is known that some strains of Burkholderia are taken at
early stages every generation from the environment (Kikuchi, Hosokawa,
& Fukatsu, 2007). Members of the genus Burkholderia are known as
significant soil bacteria, although the details of the acquisition
mechanisms of the bacteria in R. pedestris remain unknown
(Kikuchi et al., 2007; Kikuchi, Meng, & Fukatsu, 2005). However, in
agricultural lands with intensive insecticide applications, an
acceleration in the microbial degradation of the insecticide has been
observed (Arbeli & Fuentes, 2007; Singh, Walker, & Wright, 2005).
Subsequently, when R. pedestris occurs in fields heavily treated
with the insecticide fenitrothion (one of the most popular
organophosphates), some Burkholderia strains show the ability to
degrade the insecticide and demonstrate that Burkholderia confers
resistance to the insect against the organophosphate, establishing a
beneficial symbiont relationship (Kikuchi et al., 2012; Kikuchi &
Yumoto, 2013). This finding raises the possibility that
the Burkholderia observed only in Brazilian medflies may be
associated with the novel acquisition of capabilities to hydrolyse and
metabolise insecticides, thus enhancing the fitness of the host insect.
Although this hypothesis should be further tested, it might be an
example of horizontal transmission of microbiomes from the soil
associated with a demographic response to insecticides and calls for
specific experiments to understand the emergence of resistance and the
bacterial strains involved. Another example to consider as horizontal
transmission observed exclusively in
Brazil is Dysgonomonas. This member of phylum
Bacteroidetes is found on the surface of plant roots in the soil (Liu et
al., 2018) and has been described in the guts of wild specimens ofBactrocera dorsalis, a species closely related to the medfly,
suggesting that they might have been recruited from the surrounding
environment (Wang, Jin, & Zhang, 2011).
Overall, most of the prevalent genera identified exclusively in Brazil
are associated with the soil microbiome, which raises questions about
the possible functions of the microbiome in this specific locality. A
recent publication found differences in chromosomes and changes in the
allele frequency between experimental and natural populations
of D. melanogaster that were exposed to different microbiome
treatments, suggesting that a shift in microbiome composition may be an
agent of selection that drives adaptation at population levels (Rudman
et al., 2019). Here, we reveal the correlation between the microbiome
composition and the genomic structure of the populations and highlight
the importance of the host-microbiome interaction for the adaptation of
the medfly to different environments. Therefore, further research at a
population level will be required to unveil the role of the microbial
communities in the medfly.